1. Field of the Invention
This invention relates to an improved structure stabilization system for protecting structures, e.g., buildings, from effects of seismic disturbances. More particularly, the present invention relates to improvements in a base isolation system employing vertical support columns suspended by flexible elements from corresponding bases and which provide "floating" support of a structure relative to its foundation, thereby minimizing horizontal movement transmission from the ground to the structure, and to releasable interlock and damping subsystems, employable independently and/or in combination with conventional base isolation systems and preferably with the improved base isolation system of the invention. The base isolation system improvements prevent unpredicted stresses from developing in the support columns and possible tendencies of the structure to rotate relatively to the foundation while assuring that a predetermined natural period of oscillation is maintained in common for all such columns and elements, and yet affording the ability to adjust the actual length of the flexible elements as to maintain a common elevation of the support column thereby compensating for variations in ground level support of the bases, unequal stretching of the flexible suspension elements, and the like. The releasable interlock subsystem provides a single interlock between the structure and its foundation which prevents translational movement of the structure relative to its foundation despite minor forces, such as caused by wind, applied to the structure but which, in response to forces exceeding a predetermined threshold such as produced by a seismic event, automatically releases the interlock and permits the structure to "float" on the support afforded by the base isolation system. The damping subsystem impedes rotation of the structure relative to its foundation, in both the engaged and released states of the releasable interlock subsystem; it furthermore reduces the acceleration response of the system and damps lateral displacement of the structure relative to its foundation when released from the releasable interlock subsystem and thus when "floating" on the base isolation system. The damping subsystem of the invention moreover may be utilized with alternative base isolation systems which permit relative vertical displacements of opposite vertical sides of a building, such as those employing elastic isolators, to impede relative rotation of the structure in a vertical plane.
2. Description of the Related Art
Structure stabilization systems for protecting structures, e.g., buildings, from the effects of seismic disturbances are known in the art. Canadian Patent No. 872,117 in the name of the common inventor herein discloses a base isolation system which functions to minimize the transmission of horizontal movement from the ground to the structure. A plurality of bases are anchored to the foundation floor, each base supporting a plurality of cables joined to the lower end of vertical support columns which directly support the structure. While the suspension of the support columns by the cables is effective to minimize horizontal movement transmission from the ground to the structure, it has been determined that certain deficiencies exist in the prior system. For example, to accommodate both height variations in the foundation floor on which the bases are anchored which may exist in the initial construction or may occur over time due to settlement and also uneven stretching of the cables, adjustment mechanisms are provided on the bases to adjust the suspension length of the cables (i.e., the length of the cables between their respective points of attachment to the base and to the support column) thereby to equalize the elevation, or height, of the support columns associated with the plural bases. The resultant, different lengths of the cables of the plural bases, however, creates corresponding, unequal harmonic characteristics of the support cables which, in the event of a seismic occurrence, can produce unpredicted stresses in the support columns and a change in the natural period of oscillation from a predetermined value intended to be provided by the base isolation system and further may present a tendency of the structure to rotate relatively to the foundation, all of which factors may contribute to potentially destructive forces imposed on the structure and the "floating" support columns. There thus is a serious need to provide improvements for overcoming these and other defects and limitations of known base isolation systems.
While base isolation systems of the type described thus permit the structure to move relatively to the foundation (and thus to the ground in which the foundation is anchored), i.e., to "float," it is desirable to inhibit that "floating" characteristic in the absence of a seismic occurrence and, instead, to maintain the structure stable against minor forces, e.g., wind. For this purpose, it has been known in the prior art to interconnect the structure and its foundation with a plurality of breakable pins or other releasable interlock mechanisms which are of sufficient integrity to withstand minor forces which are applied to the structure (e.g., wind), but which will break or release in response to forces of a greater level, such as produced by a seismic disturbance, and thereby allow the structure to "float" in accordance with the base isolation system. A critical defect, however, can arise with such prior art releasable interlock mechanisms in that if all of the breakable pins or other release mechanisms do not function simultaneously, i.e., to break or release the structure from its foundation, destructive rotations and/or gyrations of the structure may result. There thus is a need to overcome these and other critical defects of prior art releasable interlock mechanisms.
It is also known to use energy dissipation devices, or dampers, for dissipating the energy which seismic-produced forces exert on a structure. Typically, in the prior art, a plurality of independently acting shock absorbers are connected between the structure and its foundation or otherwise rigidly attached to the earth, at generally symmetrical, spaced positions and in corresponding orientations. A problem of such prior damping systems, however, arises in that the horizontal projection of the center of gravity of the building typically does not coincide with the centroid of the horizontal inertia forces opposing displacement of the structure relative to its foundation. As a result, a net force tending to rotate the building relative to its foundation may occur, introducing gyrations that produce linear displacements between the structure and its foundation, in an amount proportional to the distance from the center of rotation to the given juncture or plane of interconnection of the structure and its foundation. Moreover, such independent shock absorbers may introduce phase differential effects which contribute to gyrations or rotational oscillations of the structure. There thus is a need for improvements which overcome such defects and limitations of known damping systems.
These and other defects and inadequacies of prior art systems are overcome by the structure stabilization system of the present invention.